1. Field of the Invention
The present invention relates to a single-chip integrated photodetector including a photodiode and a signal processing circuit, or an optical IC.
2. Description of the Prior Art
In FIGS. 1 and 2, there is shown a conventional optical IC including a photodiode 1, an amplifier 2 for amplifying an output signal of the photodiode 1, a Schmitt-trigger circuit 3 for conducing an On-Off control of the amplified signal, an output transistor 4 driven by the Schmitt-trigger circuit 3, a load resistor 5 and a stabilized power source 6. An emitter of the transistor 4 is coupled to the ground GND, and a collector of the transistor 4 is connected to a voltage output terminal V.sub.out and to a power source V.sub.CC via the resistor 5. FIG. 2 shows a cross section of the single-chip optical IC shown in FIG. 1, including three portions of the photodiode 1, an NPN transistor 7 for the output transistor 4 and a base diffusion resistor 8 for the load resistor 5.
As shown in FIG. 2, in a P-type silicon substrate 9, two n.sup.+ -type buried layers 10 are formed in the left and right side intermediate portions, and three n.sup.- -type epitaxial layers 11 are grown in the central surface portion and the left and right side surface portions on the n.sup.+ -type buried layers 10 in the above described three portions of the photodiode 1, the NPN transistor 7 and the base diffusion resistor 8. Two P-type base diffusion regions 12 are formed in the surface portions of the two left and right n.sup.- -type epitaxial layers 11, respectively, and three n.sup.+ -type emitter diffusion regions 13 are formed in the surface portions of the left P-type base diffusion region 12, the right side portion of the left n.sup.- -type epitaxial layer 11, and the right side portion of the central n.sup.- -type epitaxial layer 11, respectively. An insulating oxide film 14 covers over the surface of the P-type silicon substrate 9. Metallic wirings 15 made of a material such as aluminum are attached to the P-type base diffusion region 12 and two n.sup.+ -type emitter diffusion regions 13 of the P-type base diffusion region 12 and the n.sup.- -type epitaxial layer 11, as a base B, an emitter E and a collector C, respectively, through the oxide film 14 in the NPN transistor 7, are attached to the n.sup.+ -type emitter diffusion region 13 and the surface of the P-type silicon substrate 9 as a cathode K and an anode A, respectively, through the oxide film 14 in the photodiode 1, and are attached to the left and right side ends of the P-type base diffusion region 12 as terminals R.sub.1 and R.sub.2, respectively, through the oxide film 14 in the base diffusion resistor 8.
In the above described above, the PN junction between the P-type silicon substrate 9 and the n.sup.- -type epitaxial layer 11 constitutes the photodiode 1 having the anode A of the P-type silicon substrate 9 in the central portion, and the n.sup.- -type epitaxial layer 11, the P-type base diffusion region 12 and the n.sup.+ -type emitter diffusion region 13 constitute the NPN transistor 4 in the left side portion 7. The P-type base diffusion region 12 forms the resistor 5 in the right side portion 8.
In this embodiment, when the light L shines on the photodiode 1 while photodiode 1 is reverse-biased, a small photo-current flows therein, and the small current is converted into a voltage in the amplifier 2. Then, the voltage is fed from the amplifier 2 to the Schmitt-trigger circuit 3. The Schmitt-trigger circuit 3 has the hysteresis such that it is switched on when an input voltage becomes beyond a certain value, and, in turn, is switched off when the input voltage is below the certain value. The output signal of the Schmitt-trigger circuit 3 drives the output transistor 4, and the output signal is picked up outside from the output transistor 4. That is, a signal having a value "0" or "1" which is determined by comparing the intensity of the irradiated light with a certain reference value, is outputted from the output transistor 4 with a certain hysteresis.
However, in the conventional optical IC, since the detector part for detecting the light current of the photo-transistor 1 is composed of the amplifier 2 and the Schmitt-trigger circuit 3, the number of the elements is large, and thus their occupying area becomes wide or the chip size becomes large. Further, since the detection of the light current of the photodiode 1 is performed by comparing with the reference voltage after conducting its voltage conversion, there are many error factors and a large dispersion in its sensitivity as well as a large temperature dependence.